Photothermographic film typically includes a thin base material, such as a polymer, which is coated on at least one side with an emulsion of photosensitive and heat sensitive materials. After the film has been subjected to photo stimulation to form a latent image in the emulsion, such as via a laser imager, for example, a thermal processor is employed to develop an image from the latent image through application of heat to the film. Typically, a thermal processor raises the base material and emulsion to an optimal development temperature at which a chemical reaction of the emulsion occurs at an optimal rate. The thermal processor then maintains the film at the optimal development temperature for a required development duration to develop the image from the latent image.
One type of thermal processor is a drum processor. Drum processors typically employ a rotating heated drum having a series of rollers positioned about a segment of the drum's surface. Such rollers are sometimes referred to as “pressure rollers” and are biased against the drum. During development, rotation of the drum draws the photothermographic film between the pressure rollers and the drum, with the pressure rollers holding the photographic film against the drum so that heat is efficiently and evenly transferred from the drum to the photothermographic film.
In order to improve media throughput (i.e. the amount of imaging media that can be processed in a given time period), thermal processors typically raise the temperature of the photothermographic film to the development temperature as quickly as possible. Some types of thermal processors also quickly raise the temperature to the development temperature to ensure that chemical reactions in the emulsion proceed correctly (.e.g to ensure uniform development and a full range of optical densities). The size (i.e. diameter) of the drum is dependent on how quickly the film reaches the development temperature, the development duration of the photothermographic film, and the desired throughput of the thermal processor. Achieving an increase in throughput for a given type photothermographic film having a given development temperature and a given development duration typically requires an increase in the drum's diameter.
However, for some types of photothermographic film, the base material of a portion of the film contacting the drum rapidly expands in a cross-web direction (i.e. in a direction perpendicular to the direction of transport of the film about the drum) as the film is heated, while the base material of a remaining portion of the film that has not yet contacted the drum is not yet expanding. The difference in expansion rates between the heated and non-heated portions of the film causes the base material of the heated portion of the film to wrinkle and creates peaks and valleys in the cross-web direction of the film. The peaks and valleys absorb heat differently from the drum which causes them to develop to different densities which, in turn, produces visual artifacts.
There is a need to for an improved thermal processor that eliminates wrinkling of photothermographic film without sacrificing film throughput.